In WO 93/05966 there is described a marking instrument having a valve device comprising a diaphragm with a central hole through which extends a feed member having a feed channel for conducting marking fluid to the marking tip. The inner end of the feed channel opens through radially directed ports which are normally closed by being covered by the inner peripheral surface of the diaphragm. For opening the valve, the feed member is adapted to retract under normal writing forces experienced due to the marking tip being applied against a surface, this retraction causing the diaphragm to deflect so that it lifts clear of the ports and thereby opens up communication between the feed channel and a reservoir chamber defined rearwardly of the valve. To ensure deflection of the diaphragm, its inner edge sits against a shoulder formed on the feed member. When the force against the marking tip is removed, due to the tip being lifted from the surface, the feed member is pushed forwardly by the diaphragm which closes the ports so that the reservoir chamber is once again isolated from the feed channel. It has been found that with some types of marking fluid, when the marking tip is lifted clear from a surface against which it has been pressed, marking fluid can continue to flow or ooze to the tip despite the diaphragm valve appearing to function correctly. Although the amount of fluid is small, the collection of excess fluid at the marking tip can detract from the performance of the marking instrument and is undesirable.